Reducing Unsteady Loads on a Piggyback Miniature Submarine

Langley Research Center

A small, simple fixture has been found to be highly effective in reducing destructive unsteady hydrodynamic loads on a miniature submarine that is attached in piggyback fashion to the top of a larger, nuclear-powered, host submarine (see Figure 1). The fixture, denoted compact ramp, can be installed with minimal structural modification, and the use of it does not entail any change in submarine operations.

Figure 1. A Miniature Submarine, denoted an ASDS as explained in the text, rides piggyback on a submarine of the Los Angeles class.

The miniature submarine is denoted as Advanced SEAL Delivery System [ASDS (wherein “SEAL” signifies the United States Navy’s special-operations forces known as the Sea, Air and Land (SEAL) forces.] The ASDS is launched from the host submarine to clandestinely transport a SEAL team to a landing site, where the team performs an operation. Later, the ASDS is used to return the SEAL team to the host submarine.

Figure 2. A Compact Ramp is mounted on the host submarine hull between the rear ends of the aft pylons to modify the flow to reduce unsteady hydrodynamic loads on the stern appendages of the ASDS.

During sea trials and subsequent computational analysis, large unsteady hydrodynamic loads were detected on the stern of the ASDS during piggyback transport. It was discovered that the unsteady hydrodynamic forces and moments were associated with unsteady separated flow that was caused by the combination of a strong adverse pressure gradient on the stern of the ASDS as well as blockage of flow by a mating trunk, pylon fairings, pylon cross struts, latches, and other fixtures used for mounting the ASDS on the host submarine. The unsteady loads acted on the rudders, stern planes, propeller, stators, and stern cone of the ASDS, causing fatigue and early failure of critical components.

An investigation of flow-control modifications to reduce the unsteady hydrodynamic loads was initiated. Of thirty modifications that were considered, the one judged to be most promising was the installation of a compact ramp on the host submarine hull between the rear ends of the aft pylon pair, near the stern of the ASDS. Unlike other flow-control modifications examined, this one is not based on the concept of confronting and reducing the flow separation directly; instead, it is based on the concept of mitigating the adverse pressure gradient and moving the flow separation away from the critical stern components of the ASDS and harmlessly onto the host hull downstream of the ramp, as depicted schematically in the lower part of Figure 2.

In water-tunnel tests on a scale model, the installation of the compact ramp was found to result in reductions of as much as 50 percent in unsteady hydrodynamic forces and moments on the stern appendages of the ASDS, leading to the selection of the compact ramp as sole candidate recommended for testing in fullscale sea trials. It has also been conjectured that structural components similar to the compact ramp could, potentially, confer flow-control and load-reduction benefits in applications that involve piggyback or other external attachments to aircraft.

This work was done by John Lin of Langley Research Center. LAR-17364-1